Planning for the next generation of proton-decay experiments in the United States Page: 3 of 16
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when they interact in the detector. Soudan 1 is providing valuable
data on the rates and characteristics of such events, which are a
potential source of background to nucleon decay in the proposed
Soudan 2 experiment.
Preliminary analysis has now been completed on a total of 63 days
of data from Soudan 1, yielding the following sample of events:
190,000 cosmic-ray muon events
1,100 multiple parallel muon events (2 < Ni < 12)
395 stopping muons (0.21% of the total)
70 observed p+ decays (giving 35*4% detection efficiency)
1 upgoing muon candidate (from a v + rock interaction?)
0 neutrino interactions in the detector (- 0.5 expected)
0 nucleon decay candidates
0 slow magnetic monopole candidates
Events with multiple parallel muon tracks can yield information on the
nuclear composition of the very high energy cosmic-ray primaries which
interact in the upper atmosphere to produce bundles of decay muons.
Figure 1 shows the multiplicity distribution of these events. In
addition, the very high-energy muons which traverse the detector
occasionally roduce spectacular interactions, demonstrating the
excellent pattern-recognition capabilities of Soudan 1. An example is
shown in Figure 2.
The lack of nucleon decay candidates gives the lower limits on
the nucleon lifetime listed below. The fiducial-mass values are from
Monte-Carlo event-containment studies, using the same cuts which were
applied to the data. The detection efficiencies include a nuclear
absorption loss of 20% per pion in the parent nucleus (a remaining e+
is seen with 50% efficiency), and the > 8-tube cut on the number of
hit proportional tubes.
Decay Fiducial Detection T/branching ratio
mode mass efficiency (90% C.L.)
p + e++r 11 tons 9M% 4.5 x 1029 years
n + e+W- 13 tons 90% 5.4 x 1029 years
n + u+,- 11 tons 90% 4.5 x 1029 years
p + vA+ 16 tons 70% 5.2 x 1029 years
n + vn" 15 tons 80% 5.5 x 1029 years
p + vK+ 16 tons 35% 2.6 x 1029 vearc
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Ayres, D.S. Planning for the next generation of proton-decay experiments in the United States, article, January 1, 1982; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1112452/m1/3/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.